GENES FOR UNUSUAL ‘FLOWER
WITHIN A FLOWER’ ARE
IDENTIFIED BY UCSD SCIENTISTS

Gertrude Stein once wrote that
a rose is a rose is a rose. But not all roses consist of mundane
single flowers. One of the more unusual curiosities of the plant world—among
roses and other flowers—is a flower within a flower within a flower.

The abnormality has long
puzzled geneticists, who have sought for years in vain for the genes
responsible for this condition. But an important part of the mystery
has been solved by a team of biologists at the University of
California, San Diego.

In a cover story in the May 11th
issue of Nature, the scientists report that they have
identified a trio of genes that produce this flower within a flower,
one of the earliest-recognized abnormalities in flowers.

Double
Arabidopsis flower

First described more than 2,000
years ago and initially referred to as a "monstrous flower,"
the abnormality is now called the "double flower" and is
prized within the flower industry for its attractiveness. Many roses,
camellias and impatiens, as well as a host of other plants produce
these double flowers, which are grown from plant cuttings because the
plants, having lost their reproductive organs, are effectively
sterile.

Normal Arabidopsis
flower

Plants do not normally produce
double flowers in the wild and, in their study, the UCSD biologists
discovered why. Only when three virtually identical genes within the
plant’s cells are all mutated, they found, is the result a flower
within a flower within a flower, a repetitive process that continues
indefinitely—or at least until the smallest organs of the flower can’t
be detected.

"They endlessly reiterate
flower organs, they just keep on going and going," says Martin F.
Yanofsky, a professor of biology at UCSD who headed the research team,
which included

Max-Planck Institute for
Breeding Research in Koln, Germany also participated in the study,
which was financed by the National Science Foundation and the National
Institutes of Health.

Normal
flowers consist of a series of four rings or "whorls." The
outermost whorl is made up of sepals, the green leaf-like organ that
normally surrounds the flower bud before it opens. Inside the sepals
is a ring of petals, then a ring of stamens, the male reproductive
structures, and at the center are the carpels (often referred to as
the pistils), the female reproductive structures.

When the three genes found by
the UCSD scientists are all mutated, the petals, stamens and carpels
are all converted into sepals, resulting in the double-flower
character. "Because these genes are necessary for petals, stamens
and carpels to form in a normal flower, they are master regulators of
flower development," says Yanofsky.

The important role of these
three genes had long been hidden from geneticists, because all three
genes are virtually identical, meaning that the UCSD researchers had
to mutate each one of these genes and then combine the three mutations
into a single plant. This technique, referred to as "reverse
genetics," took years of effort in the laboratory, but finally
paid off in the discovery of the triple mutant in the weed plant, Arabidopsis,
which has long been used as a plant model by geneticists.

"This was a heroic
undertaking, as three genes had to be knocked out in order for the
function to be revealed," says Detlef Weigel, an expert on flower
development who is an associate professor at the Salk Institute for
Biological Studies. "This paper will make it into the
textbooks."

Botanists proposed hundreds of
years ago that the sepal, petal, stamen and carpel organs that make up
a typical flower represent modified leaves. But despite the rapid
progress that many researchers around the world have made over the
past decade in isolating key flower-control genes, they have not been
able to convert leaves into each of the flower organs.

"This discovery may add
the missing piece of the puzzle, as we now know that these genes are
necessary for the formation of the different flower organs," says
Yanofsky. "It will now be interesting to directly test this idea
by turning on these genes in leaves, where they are normally turned
off. This could well make for some very interesting new plant
varieties that have, for example, colorful petals replacing the normal
leaves."